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Real Road Rat

540 streetable horsepower on pump gas, without a blower. You really have to love big-blocks

Seth Millhollin May 19, 2003

When it comes to affordable street performance engines, many enthusiasts are making serious horsepower by using small-blocks with some sort of centrifugal supercharger or by dumping in heavy doses of laughing gas. But we couldn't help but wonder if it were possible to assemble a big-inch Rat motor that shells out more than 500 ponies and 500 ft-lbs of torque, runs on low-octane pump gas, and most importantly, won't cost and arm and a leg to build? It seems that maybe some people are under a spell when it comes to thinking that a big-block can be assembled for a reasonable price tag and still offer big performance in return. Do those same folks think that big-blocks are a thing of the past? We hope not.

It's true that the new generation of crate motors offer a good starting point for potential horsepower in both big- and small-block configurations. And while you can spend a chunk of money trying to fill out your horsepower recipe with a 540-inch mega Rat, there's still a ton of usable street power to made from a less costly 454 starting point. With the help of a GM Performance Parts block, a stroker combination from Scat Engineering, and heads and induction components from Holley, that's exactly what we are going to demonstrate with this affordable build up.

We didn't start with a "crate motor" per se, but we did use a Gen 6 block as a basis for our streetable stroker and went from there. As typical as we are in comparison with every other "car guy" out there, we just couldn't stay happy with the stock displacement, so we went a little bit further now, to get a lot more out of it later.

We opted for a stroker crank from Scat Engineering, along with a set of I-beam rods. We punched the bore out by .030 and filled the holes with a lighter weight forged piston from SRP. The final displacement of this beast ended up at 490 inches. This still isn't as big as the most common crate motors like the 502s, but the power that will be produced will certainly be on par with those costly bigger engines.

As always the cylinder heads are really what makes the difference in power. Choosing a proper casting for a street motor can often be a task of it's own. Finding the correct head to flow good numbers from the start and make usable power is no easy challenge. One thing is for sure, a good oval port is really what this power plant needed.

Holley Performance Products had just the ticket, their large oval port aluminum head. This cylinder design provides immediate response from about 1,500 rpm, and continues to pull all the way to 6,000 rpm. This is the absolute best rpm range for an honest street motor. This will provide a ton of low-end power that will throw you in your seat the second you decide to stand on it.

A healthy street cam would also promote the low-end pull, while we looked to optimize the benefits of a late-model motor. Using the stock hydraulic-roller lifters and retaining plates is a wise choice, but at the same time using an aftermarket bumpstick is the best way to get the valve lift and duration numbers you're after. With that in mind we used GM Performance hydraulic-roller lifters with a Lunati camshaft that had a duration of 240-250 degrees at .050. The rest of the valve train was also from Lunati, like the roller rockers and pushrods.

We finished off the top with a Stealth dual-plane manifold from Weiand. Using a dual plane will also allow the motor to run at low rpm. Too many people think that a single plane is superior to a dual plane, but without a lot of compression or a high rpm motor, the single plane can cause a flat spot until the engine reaches a higher rpm, kind of like a stumble sensation.

After careful assembly, we tried out a few different carburetors when the engine was on the dyno, all of them from Holley Performance Products. In this we had the option of saving a little gas with a vacuum secondary, or getting a little more power with a double-pumper. In the end it was easy to settle, the 850-cfm double-pumper performed awesome. It idled much nicer and was more responsive, and of course made more power overall.

To build and dyno this engine we took a short trip over to our friends at Vrbancic Brothers Racing in Ontario, California. Both George and Bob have decades worth of experience building and racing Rat motors. And with our goal to put together a relatively inexpensive big-inch big-block, their expertise was invaluable. The short story here is using quality components that are matched together and being careful during the machining and assembly processes yielded us a stout motor that is sure to set us back into our seat cushions. With parts from Holley, SRP, and Milodon, coupled with Wayne's Auto in Riverside doing the machining steps and the Vrbancics putting her all together, we were sure that we'd be getting good numbers when we hit the dyno and reliability when it hits the streets. Check it out, and we're sure you'll agree.


Here are the main components of our affordable 490-inch Rat. A new crankshaft and a new set of rods from Scat, a new set of SRP forged pistons, and a variety of things from Holley Performance Products. This set of Street Avenger cylinder heads is one of the cornerstones of this buildup.

We met up with our GM Performance Parts 454 block when it was being machined at Wayne's Engine Supply in nearby Riverside, CA. The technicians at Wayne's did an excellent job by getting this block dead square.

The rotating assembly was spun balanced to perfection. The rods and pistons were weighed to make sure they were within tolerance, and all were right on the money.

Before assembly could begin, everything had to be checked. Here George Vrbancic carefully measures the number one journal of the crank, and the first main cap. This is to check for the proper amount of clearance, which is about .003 inches.

We also did the middle journal and the rear journal. They were all perfect.

The piston rings were also filled at this time, using this trick little tool. Most people don't have a ring filer like this, but just about any type of small file will work.

We ground off .005 inches of each side of the ring, just for starters. Then went ahead and got it perfect.

Here are the rods and pistons as they were being cleaned. Every rod and piston was cleaned by hand in the solvent tank and then blown out with an air nozzle until they were absolutely spotless.

The block was rinsed thoroughly with a mixture of engine degreaser and simple green detergent. When it was finished it was clean enough to eat off of.

With everything prepped and ready it was time to do the assembly. With the main bearings in and greased it's important to make sure that the crank is set straight down in the main bearing saddles.

Using a little bit of grease under the head of the bolt, install the main caps. When you torque it down be sure to work from the center of the engine forward.

This is important to remember when using these late-model blocks. If this block were used in a truck it would have two oil coolers lines going to the two plugs. Since it's a street engine you plug the two holes and use the adapter for a standard oil filter. If you're concerned about high oil temps, you can plumb in a filter.

Since the SRP pistons we used were for full floating rods, installing them was simple. Just shoot a little oil, line up the rod and push the wrist pin through. The Spiro-locks used can often times be difficult, but they go in securely once you get the hang of it.

With the rings on the pistons, the assembly was ready for install. Using a ring compressor, set the piston in the hole and line it up correctly.

When in straight, tap down on the piston and seat the rod on the crank.

When the rod is seated install and seat the cap by hand.

Once the rod cap is centered, torque it down starting with half of the total torque spec.

Here is a shot of the valve train that was used. A set of roller rockers, an hydraulic-roller camshaft, and a set of hardened pushrods, all of which are from Lunati. The lifters are from GM Performance, and are a stock hydraulic-roller lifter.

A thick assembly grease is what really saves a cam during break-in. It was applied to every lobe of the bumpstick as it was carefully maneuvered into the block.

On the Gen 6 big-blocks a cam retaining plate has to be used. The cams for these motors will have a protruding face on the front of the front journal. The plate will fit over that and bolt to the block, thus retaining the cam from walking forward.

A stock timing chain and cover also had to be used on this new-age block. With number one cylinder on TDC, we installed the cam at zero by lining up the dots.

The cover no longer uses a gasket, for it has a rubber O-ring incorporated in the outer edge that seals against the block.

A Milodon high pressure/high volume oil pump and pick-up was used for superior oil distribution. Using a 5/8 open-end wrench the pick-up was in tapped into place; don't forget to reinstall the bolt.

With the oil pump assembled we set it in place and installed the Milodon windage tray. These new blocks come stock with a windage tray, so this one fit without changing the stock studs.

Using the Fel-Pro gasket set, we installed this one-piece oil pan gasket. These one-piece gaskets are very nice to use and fit like you would not believe. You still need to make sure you glue down the corners, though.

The stock oil pan did have to be cleared a little bit around the pan rails. The stroker crank came out just far enough to hit the side. Using a torch and a ball-peen hammer, the pan rails were slightly notched. Once painted and installed you couldn't tell at all.

The new lifters that are in these motors are a little different than usual. They have flats on both sides and the lifter will only fit one way. These plates are used instead of gates, and will sit over the top of the lifter.

This plate bolts to the bosses in the middle of the lifter valley. The fingers that sit against the plates will keep tension on the plates so they cannot come off.

The Holley Avenger cylinder heads were set directly on the dowels. We were careful not to drag them across the gasket. Fel-Pro head gaskets also came in the set.

When the heads are in place they should be torqued from the center out. Use a little bit of grease under the head of the bolt for proper seating, and gradually increase the amount of torque. Don't go straight to the desired torque spec, for this can stretch out the threads of the bolts.

Use a spot of grease on the tips of the pushrods when putting them into place.

Use of a hydraulic-roller lifter allows you to adjust valves just like a standard hydraulic, with zero lash.

The intake manifold gaskets actually sit in a slot in the corners of the head gasket. These have rubber O-rings around the ports and require no silicone, though it still must be used on the two edges.

With the new Weiand Stealth manifold in place the bolts were tightened down working from the center out. You should get a nice even squish of silicon on both sides.

Some of the final touches were this aluminum water pump from Weiand...

...and this high-volume fuel pump from Holley.

Here is our masterpiece completely assembled. This is the first of a series of carburetors, measuring 770 cfm from the Holley "Street Avenger" line. The highly polished chrome valve covers are also from Holley and were an exceptional way to finish off the engine. It was put on the dyno using an HEI distributor at 35 degrees of total timing on 91 octane unleaded straight from the gas station.

Here is our engine behind the glass and in the hot seat. It sounded awesome sitting in this spot when this picture was taken. If you can't read the computer screen, the tachometer is nudging 5,446 rpm and on its way up. A few carb changes were the only real tuning we did, leaving us to believe that there is still some power left in this 490-inch Rat. With the results we received, however, we were more than happy.


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